The 3' region of the thiamine biosynthesis gene THIC contains a riboswitch aptamer (i.e. a RNA motif adopting alternate structures depending on the binding of a metabolite) sensing the thiamine pyrophosphate (TPP) concentration. In some plant species the TPP riboswitch has been also found in THI1, another gene of thiamin biosynthesis.The TPP riboswitch, whose structure has been confirmed by crystal structure resolution (Thore et al. 2006), consists of five helices, termed P1 to P5, and h......